Project Summary/Abstract Up to 10% of the global population is affected by neuropathic pain caused by lesions or disease to the somatosensory nervous system. A common symptom of neuropathic pain is dynamic mechanical allodynia, a condition in which light brushing of the skin is perceived as extremely painful. Many patients that suffer from mechanical allodynia do not respond to current treatments. In neuropathic patients, it is well-established that the activation of myelinated low-threshold primary afferents is required for mechanical allodynia. However, there is significant diversity in these afferents and the specific primary afferent subtypes involved in driving allodynia are unknown. It has also been shown that lamina 1 spinal projection neurons, which receive input from primary afferents and project to the brain, become sensitized to mechanical stimuli after neuropathic injury and are thought to be involved in mediating allodynia. Therefore, I hypothesize that the activation of specific low- threshold mechanoreceptive primary afferent subtypes provide greater input to lamina 1 spinal projection neurons and cause aversion in mice after neuropathic injury. In Aim 1, in nave and neuropathic mice, I propose to use a spinal cord-nerve-skin ex-vivo preparation to activate specific subtypes of low-threshold primary afferent using optogenetics while recording from back-labeled lamina 1 spinal projection neurons. In these initial studies, I will examine three different low-threshold afferent subtypes that respond well to light brushing of the skin, including A? low-threshold afferents, A? rapidly adapting afferents with lanceolate endings, and A? afferents with circumferential endings. As a potential electrophysiological basis for allodynia, the results will determine which subtypes of low-threshold primary afferents give greater input to lamina 1 spinal projection neurons after neuropathic injury. In Aim 2, in a behavioral model, I will employ an LED-lit place aversion experiment to activate specific low-threshold afferents using optogenetics in nave and neuropathic mice. Here, the results will determine which low-threshold primary afferents contribute to aversive behavior after neuropathic injury. In conclusion, it has been shown that blocking all low-threshold primary afferents alleviates mechanical allodynia in patients, but the specific subtypes of afferents involved in allodynia are not known. This knowledge may lead to therapeutics that block allodynia at its source while maintaining the majority of cutaneous sensory afferents.